Role of Ascorbate and Protein Thiols in the Release of Nitric oxide from S-Nitroso-Albumin and S-Nitroso-Glutathione in Human Plasma

In this work we investigated the stability in aerobic plasma of two naturally occurring S-nitrosothiols, the S-nitroso adduct of serum albumin (S-NO-albumin) and the S-nitroso adduct of glutathione (S-NO-glutathione). In contrast to their behavior in physiological buffers, in which they are stable, in plasma these S-nitrosothiols showed a slow but continuous release of √NO. In the presence of red blood cells, the √NO was quantitatively oxidized to NO3− with stoichiometric formation of methemoglobin. In the absence of red blood cells, the principal oxidation product was NO2− with small amounts of NO3− (about image of the amount of NO2−). The release of √NO was also proven by spin trapping experiments with 2-(4-Carboxyphenyl)4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide which, when added to plasma in the presence of S-NO-glutathione, was transformed into 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl. Both dialysable and nondialysable compounds are involved in the release of √NO from S-nitrosothiols. Ascorbate and the thiol group of serum albumin are the plasma components mainly involved in the release of √NO, while endogenous image-cysteine and glutathione play a minor role due to their relative low concentrations. However, in contrast to the thiol-dependent release that is known to induce the formation of disulfides, the ascorbate-dependent release of √NO from S-NO-glutathione resulted in the formation of free sulfhydryls. Our results suggest that in plasma the √NO release from S-NO-albumin and S-NO-glutathione may be regulated by heterolytic NO+ transfer and reductive activation to √NO, rather than by homolytic decomposition of labile S-nitrosothiols. Copyright © 1997 Elsevier Science Inc.